This invention relates to electrostatic precipitators and, in particular, to a way of greatly improving the operating efficiency of electrostatic precipitators.
It is usually desirable and frequently required to remove impurities entrained in a stream of gas. For example, Federal anti-pollution regulations require removal of particulates, such as fly ash, of the type produced in enormous quantities in combustion gases from the production by utilities of electricity from steam produced in boilers fired with fossil fuels. Large coal-fired boilers, for instance, produce up to 4,000,000 cfm (114,000,000 1/min) of flue gases at temperatures ranging to 1000.degree. F. (538.degree. C.) which entrain large quantities of fly ash and other particulate waste, 1000,000 lbs./hr. (45,000 kg./hr.) of such particulates being a representative quantity. Inasmuch as the pulverized coal burned in many boilers is finely divided when burnt, the resulting ash particulate is very small in size and is difficult to remove from the effluent gas. Recently established air pollution codes commonly require continuous removal of more than 99% of the particulate efflent.
Of several known types of gas cleaning apparatus, electrostatic precipitators have been found to be the most satisfactory for cleaning the very hot and often corrosive flue gases from fossil-fueled boilers and other combustion devices. In 1910, Frederick G. Cottrell described one of the first electrostatic precipitators in a United States Patent, and his name is used as a generic name for a class of electrostatic precipitators. Modern large Cottrell-type electrostatic precipitators usually have several chambers, each chamber being several precipitator cells wide and several precipitator fields deep. The combined cell and field dimensions define commercially available modules, in one type of which total effective cell widths range up to 150 ft. (46 m.), total effective field lengths range up to 45 ft. (14 m.), and effective heights range from 10 ft. (3.0 m.) to 45 ft. (14 m.). Each module comprises plate collectrodes that are relatively closely spaced, e.g. about 9 inches (23 cm.), from each other and extend parallel to the flow of gas to be cleaned in the module to define gas flow passages, and high voltage discharge wire electrodes positioned along the passages between the collectrodes. The wire electrodes are highly charged, often to -40,000 V.D.C., and the collectrodes are grounded. The electrical fields between the electrodes and collectrodes ionize (charge) the gas and particulates flowing through the chamber and the charged particulates are moved by electrostatically generated forces to the relatively lower potential collectrodes where they are retained by electrostatic attraction.
As a layer of charged particulates builds on the collectrodes, the potential (voltage) between the high voltage electrode and the particulate-coating on the collectrodes decreases and gradually decreases the efficiency of the precipitator correspondingly. After a layer of particulates has accumulated to a point (which varies depending on the design and use of the precipitator), it is necessary to clean the collectrodes to remove the collected particulates. Collectrode cleaning is usually done in commercial preecipitators by "rapping," a procedure involving application of a mechanical shock to the collectrodes while normal gas flow and high-voltage powering continues. Rapping causes primarily relatively large agglomerates of the particulates to fall into a hopper below the precipitator, but other particulates, for example, individual particles or small agglomerates, become reentrained in the gas and are carried downstream with the normal gas flow and must be recharged and recollected by a downstream field of the precipitator, lest they flow out with the gas. However, at the end of the precipitator, some of the re-entrained particulates escape through the outlet and are discharged through the slack to the atmosphere. Relatively long gas flow passages, numerous fields in series and relatively low gas flow rates are typical features of modern, high-efficiency precipitators that are incorporated to minimize the percentage of particulates finally re-entrained with the discharged gas. Those current design practices make today's precipitators very expensive to construct, operate and maintain.
The following example is given to illustrate more fully and quantitatively the progressive collection of particulates along the gas flow passages of an electrostatic precipitator which is built and operated in accordance with current commercial practice in that collectrode cleaning (rapping) takes place while gas flow continues at normal velocity. In modern precipitators, the great majority of particulate is charged and precipitated on the collectrodes within a few feet of entering the first field of precipitation. Often, about 50% of collected particulate is deposited in the first row of hoppers under the first field. It is believed that the major part of that loss is due to re-entrainment of particulate caused by collectrode cleaning. Assuming for this example that a representative loss due to electrode cleaning is 40% of particulate from each field of a precipitator, Table I then demonstrates that eight fields are required to collect 99.6% of the particulates. In cleaning a representative gas stream in which there is entrained 100,000 pounds per hour of particulates, Table I demonstrates the greatly diminishing effectiveness of successive fields. The first field collects 50,000 pounds of particulate per hour; the eighth field collects (and loses) 390.6 pounds per hour.
TABLE I __________________________________________________________________________ Pounds Per Hour of Particulate __________________________________________________________________________ Lost To Precipitated Agglomerate Re-entrainment Total Lost Total Entering on Dropped Into During Rapping to Percent Field Field Collectrodes Hoppers Collectrodes Next Field Collected __________________________________________________________________________ 1 100,000 90,000 50,000 40,000 50,000 50.0 2 50,000 45,000 25,000 20,000 25,000 75.0 3 25,000 22,500 12,500 10,000 12,500 87.5 4 12,500 11,250 6,250 5,000 6,250 93.7 5 6,250 5,625 3,125 2,500 3,125 96.9 6 3,125 2,812.5 1,562.5 1,250 1,562.5 98.4 7 1,562.5 1,406.2 781.2 625 781.2 99.2 8 781.2 703.1 390.6 312.5 390.6 99.6 TOTALS 99,609.3 390.6* 99.6 __________________________________________________________________________ *Total Loss to Outlet
There have been various suggestions in the past for minimizing the loss of particulates to the stack of an electrostatic precipitator during rapping. For example, it has been proposed to stop or slow the flow of gas through all or part of an electrostatic precipitator during rapping, thereby reducing the carryout from the precipitator of particulates that become re-entrained in the gas. Among the ideas proposed for that purpose is the provision of doors or louvers at some point or points in the precipitator that can be closed during the rapping cycle. Relatively complicated duct and louver systems have been devised for periodically interrupting or reversing the gas flow through individual chambers of an electrostatic precipitator during a rapping cycle in the chamber.
The arrangements that have been proposed have never been generally adopted for commercial electrostatic precipitators. The difficulty of constructing, maintaining, and operating louvers or other door arrangements in the hot, corrosive environments encountered in electrostatic precipitators has undoubtedly been one of the reasons for the lack of commercialization of such ideas. The proposed arrangements also were not susceptible of completely stopping gas flow through the precipitator, or part of the precipitator, and, therefore, substantial losses in efficiency due to leakage were unavoidable. Accordingly, the tendency in the design of electrostatic precipitators has been to increase the number of fields and to program carefully the rapping cycles among the various fields, as well as to compromise in the design of the rapping system to minimize particulate re-entrainment, but only to the detriment of effective collectrode cleaning.
Those design approaches have permitted total precipitator efficiencies adequate to meet standards required in the past. However, the requirements that are now being imposed on precipitator efficiency can be met based on presently existing technology only by very substantially increasing the size of the precipitator, and, therefore, the costs of building, operating and maintaining it, to increase the number of fields, reduce gas flow velocities, and make additional compromises in rapping procedures.